A question on ELU (“Sassanian” vs. “Sasanian”) brought up the fact that the name of Sāsān has often been spelled in English with a double S in the middle: "Sassan". (The same goes for related words like Sassanid.) This is a bit mysterious because as far as I can tell, the name is not pronounced with long /sː/ in Persian (nor is there any evidence that it ever was—if I'm wrong about that, though, please let me know!).

I am wondering what kind of precedents, if any, there might be for this spelling in Latin or in Greek. I tried to research this a bit for my answer to the linked question, but I didn't find out much; however, I am reproducing my findings in this question.

The etymology section of the Oxford English Dictionary entry for the English word Sassanid seems to indicate that -⁠ss⁠- spellings of related words existed in medieval Latin: supposedly, Sassanid comes from "medieval Latin Sassanidae plural, < Sassan, Sasan". But I haven't been able to find any more sources that explain why -ss- might be used in Latin Sassanidae. I know that medieval spellings were often variable, but I don't know if the use of -ss- here ought to be explained as just a meaningless alternative to -s-.

Varro's answer here suggests that Greek -σσ- (rendered in Latin and English transliteration as -ss-) may have been used in the transcription of non-Greek certain names to represent some unclear difference between sibilants in foreign words and the sibilant /s/ sound found in native Greek words. For example, it seems the use of -ss- in the modern English spelling of the word "Messiah" can be traced back to Greek.

The main component of the name Farn-Sāsān is the word ssn/Sāsān, which was in widespread use in Ancient Parthia, including the Nisa texts. The etymology of Sāsān is controversial. D. N. McKenzie and V. A. Livshits derive it from Old Iranian *Sāsāna, "Defeating Enemy". [...]

Recently, M. Schwartz has proposed another etymology of Sāsān. He sees a continuum from the ancient Syrophoenician form *Sasm to the Late Antique Greek and Aramaic form Sæsæn/Sesen. But his alleged Parthan Sesen for ssn is not convincing, for linguistically no links are discernible between *Sasm or Sæsæn/Sesen and Parthian ssm. Moreover, the occurrence of the non-Persian name ssn spelled in Greek Σασαν or Σασάνης in the ŠKZ inscription clearly implies that this is actually the Parthian name Sāsān.

A later section in this chapter titled "Sāsān in Roman and Armenian sources" seems to quote ancient texts in Classical languages about Sāsān, but I haven't found information about the original spellings used. The texts mentioned are "Agathias II, 27, 1-5: 1" and "Synkellos [p. 440.11-441.2 (p. 677.11-678.7 €SHB)]".

I would appreciate it if anyone could tell me more about the way this name was spelled in Greek and Latin of various eras.

Another thing doubled consonants can indicate, besides consonant length, is shortness of the preceding vowel. My hazy memory is that this goes back at least to some period of vulgar Latin — which did have contrastive consonant length (unlike eg English), but also had the association of short vowels with that, so in regular spelling doubled consonants indicated both, and in loanwords could indicate either. Another thing that -ss- vs -s- has come to indicate in some Romance languages is voicing; but that iirc is a much later development, so unlikely to be relevant here.
– Peter LeFanu LumsdaineMay 12 at 8:16

I would think that the reason in English why any consonant would be doubled would have to do with its intended pronunciation. Chiefly here would be that double consonants shorten the vowel that precedes them, so sass-an vs. s(ay)-san.
– Media Matella Lucretia Flores2 days ago